Telephone system



March 5, 1946. w. G. STANLEY ET AL TELEPHONE SYSTEM.

yFiled My 12, 1945 5 Sheets-Sheet 1 ohm. 28. 23m

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. 2n; EME uo $355 E lNvENToRS WML/1M 6 STA/usf Fea/wc H 6765 BY A E ATroR' .CD26 Soru March, A1946. l W, G. STANLEY ET AL 2,395,916

TELEPHONE SYSTEM 1943 5 lsheets-sheet 2 Filed vMay l2,

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m22 Pr INVENTCRS WILL/AM 65771/1452 Fia/YK /7 515 A'I'I'ORN EY March 5, 1946. w. G. STANLEY ET AL TELEPHONE SYSTEM Filed May l2, 1945 5 Sheets-Sheet 3 l v March 5, 1946. w. G. STANLEY ET'AI. 2,395,916

A TELEPHONE l SYSTEM i Filed May l2, 1943 5 Sheets-Sheet 4 ATTORNEY March 5, 1946. W STANLEY ET AL 2,395,916

TELEPHONE SYSTEM Filed May l2', 1943 5 Sheets-Sheet 5 f /N cycLEs INVENTORS.

WML/M1 G. 5 Tf1/ufr Ffm/wc f7/3f" l BY ATTORNEY Patented Mar. 5, 1946 TELEPHONE SYSTEM William Gibson Stanley, New York, N.-,Y., and l Frank Augustus Leibe, Quakertown, Pa., assignors to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Dela-V Ware 'Application May 12, 1943, serial No. 486,630 p 1.5 Claims.

This invention relates to a ringer for use in connection with telephone switchboards, whereby the customary low ringing frequency of around 20 cycles is transformed into a higher frequency lying within the voice range, such as 5001er 1000 cycles. This higher frequency can readily'pass over transmission lines, along which apparatus such as repeaters or carrier telephone circuits may be placed, due to the fact that lsuch pieces of apparatus are designed to pass frequencies Vof 500 or 1000 cycle, whereas they frequently are not capable of efficiently passing a frequency as low as 20 cycles. Suitable conversion apparatus at the receiving end Vof the transmission line reconverts the 500 or 1000 cycle ringing current back into the usual 20 cycle current, for operating switchboard or other termina1 apparatus, customarily designed for` such relatively low frequency ringing current.

One purpose of this invention is to provide an assembly of various cooperating units 'for insertionV at the sending end of a transmission line, between the operators ringing key and the line proper. At the receiving end, a similar unit is inserted between the line proper and the drop or other indicating device employed. l

A further object of this invention is to provide,

in a single assembly, apparatus of the type abovedescribed, which will be capable of automatic twoway operation, so that such apparatus willrwithout manual switching, perform the frequency conversion function at the sending end of the line, in a direction from low to high frequency and at the receiving end ofthe line, will perform the frequency conversion in the opposite direction.

Yet another purpose of this invention is to proA vide apparatus of the type above-described, which can easily and quickly be inserted into a, trans mission line, at both sending and receiving ends thereof, and Without causing undue loss or weakening of speech frequency currents.

A still further purpose of this invention is to provide, at the receiving end of a transmission line, apparatus which will be able to differentiate between vfrequencies of 500 or 1000 cycles as ocas oscillators, amplifiers anddetectors and it also includes a power supply. unit affording the po' tentials and currents necessary for the operation of such tubes, as well as for the operation of recti fiers, relays'and other` elements of the assembly.

The device of this vinvention affords a ringing lcurrent for transmission purposes, having a frequency of 500 or 1000 cycles and optionally modulated at about cycles, this ringing. current being demodulated at thereceiving end and determin- Y ing the application of a 20 cycle current thereat.

curring at random intervals during the course of 1 speech transmission, and a 500 or 1000 cycle current intentionally used for ringing purposes, or any other frequency or narrow band of frequencies used for signaling purposes or the like, whereby undesired or false operation of the ringing or .signaling elements of such receiver during voice transmission is avoided. f

The complete ringer assembly of the present invention is a device employing electronic tubes The construction and operation of this apparatus may be better understood` by reference to the drawings herewith, where:

Fig. 1 is a block schematic diagramL of ay circuit embodyingV two complete ringer assemblies according to this invention, klocated at the respective ends of atransmission line. 1 Y

Fig. 2 is a detailed schematic diagram o f the 20 cycle rectifier unit employed' in a ringer assembly.' kaccording to this invention.

Fig. 3 is a. detailed schematic diagram ofthe oscillator unit, similarly employed.

Fig. 4 is a detailed schematic diagram ofthe l receiver unit, similarly employed.

Fig. 4(a) shows an alternative method vof connection of' the feed-back circuit between tubes Vland V3of Fig. 4.

Fig.y Mb) shows an alternative'form ofl filter Y 'which may be used in one portion of the circuit of Fig. 4.

Fig. 5 is a detailed schematic diagram of apower supply unit `which may be\ employed with the ringer of this invention. y

Fig. 671s a detailed schematic'diagram of a sin gle complete ringer unit, exclusive of the power supply, according to this invention, as used either at the sending or the receiving end of a transmission line.

Fig. 7 is a graph showing the frequency-voltage relations in one filter element of this invention.

In all the figures of theY drawings, certain relays have the armatures thereof shown separatedrby some distancefrom themagnet'thereof, for the sake of greater clarity in illustrating the circuits. However, such armatures are sh own'in alignment with the magnets of the particular relay towhich they `Vbelong, so that the identification of such relay with its own particular armatures is unmistakable. I

Referring now to Fig.v 1, there is here illustrated aV complete transmission circuit, with respect Vto ringing signaling, and including a portion ofthe associated switchboard elements at both. the sendv ing and the receiving ends. -The elements" shown the right hand portion of the operatorjsringing key |02. The position of this ringing keyplaces upon the input of the ringer gthefapproximately 20 cycle ringing current available at -theswitche board, as indicated by the legend appended to the upper richt hand portion of ringing key |02.

At |03 is indicated a sourcenof20 cycle current, shown in detail in Fig. 3.` At the-fsending end, this source of 20 cycle current is not directly. connecteditof'the :line 4at thelsender, because vthe output .circuit :thereof remains -open atlthe contact points -of `relay 1101. armatures 1| and?, the :action `of' lwhich :relay .will later tbe described At 104 is indicated :a receiver, show-nin detail in i'Fig. 4. which :remains in ran inactive state at the sending station. Relay |0l Vlis .excited Lonly bythe output `,of receiver |04 and accordingly, when fsending, .fall :the armatures -of this relay remainfi-n the respectiveiopempositonsindicated. This allows 5the 20 cycle current from fthe operators key :to .flow v.rthrough armature .sl fand contact .La of frelay rldll, v.as @well Aas through tarmature 2 and contact d of the same '.relay. The .20 cycle .Qurrent'thusireaches rectifier l||l5.fshoWn in detail in Figc. lThis rectiergiszat a-.llftiines'bridged lacross :the-transmission fline, :but is iso designed as to present a comparatively :high "impedance Ato *voice frequencies, .so :as #to lcau'se :only afnegligibleiloss to Itheyoice:currentionizhevline. How ever, the rectifier is vdesigned tohave ahigh ac,- ceptance :for the20 :cycle iringing current, fimpressed upon it at :the sendingend.

`As later explained ginY ,detail in connectionewith Fig. 2, rectifier 1:05 .acts-toirectify ,the 20 l.cycle current and gives .fan fcutput fof :direct :cur-'rent ,tort-he winding mf -relay |08. Afl'heiexcitation of relay |08 causes armature I of lthisarelayito :move downwardly .ato .contact .cwandarmature s2 -to mov@ upwardly to y'contact z'.` ffl'hese contacts just referred to are connected to the output .of oscillator y.lgilfyalso shown 1in Adetail ;in lig. 3. thisroslloftor 'zS set :into Aaction zby litho tr-ans.- ference of armature 3 of Arelay 4.0.8- from contact point k @to contact point-fi, as dater described :in detail in connection with fFig.

:Elize oonneotionfof oscillator ma f-,to the :trans mission :line fis llikewise accompanied by the dis.- connection of the line from other ,portions -of the .sending rincer, due tothe opening' of :oorltacii :and -71 atv armatures l and .2 respectively, of frelay |08. (oscillator |06 is arranged to producefa current flying inzthevoice frequency range, such 5.0.0l or f 1000 .cycle .and Voptionally .modulated --at 20 .f cycle, `:as hereinafter :explained connection -;With gFig. Accordingly, rthe transmission `line proper lis mow supplied with the voice frequency .-current Vproduced by :oscillator m6.. 1

Referring now to the right end or receiving portionof Eig. :L eoscillator |06 is '-,there disconneotedfifrom :the eline, :due to lthe onen-'positions assumedtbyarmaturcs il and offrelay IBB.. fitof- 1ay.;|08:is, of :conseguira-:excited .fbyvrectier 1.1.05.. dueitothe act rthatzno :29 cycle s.current is lprescrit atactfb ,and-,from contact d to contact e, respectively.

L Contacts b Iand ,e are connected to 20 cycle source |03 at .if/11e Vreceiver and the closure of armature 3 ofvrelay |01 against contact c, ini- -.tiatesthe=operation of 20 cycle source |03. The output of 20 cycle current derived from source |03 traverses ringing key |02 which, in the nonfringing .position chown, causes zthe 120 Acycle current ultimately :to reach .drop 3|0.|, :or other suitable indicatingdevice providedlatzthefreceiver and designed ."tofbe ioperatedby :the customary #20 cycle ringing current.

l:With reference 'to .the .description of the other figures l.of the drawings, zit .is fto i-,be .noted that Certain fvalues ,of electrical .constants are given in :connection with yvarious elements thereof. llt is to :be understood that :such values are merely illustrative .fand may be subject to considerable alteration, aswill .be apparent to .one 'skilled in the fart. rliikewise V,it is to be 4understood that certain fspooio :frequencies kherein mentioned, e. g. zocycle, 5Q0 cycle yand 1000 cyc1e,.ar.e rmerely chosen as i1lustrative,and that if oneof these freouenoiestakes on different value, there will be needed :corresponding ohangesn the velectrical constants of certain elements Vof 1the circuit, such as ,inductances and capacities.

Reference fis :now :modo :to fris. f2 fof tho fdraw ings. illustrating the :2,0 zovole raotsr in .-dsnil, and its interconnection toothor elements of :the ringer assembly.

.Across the lino looming from :tho switchboard is :bridged a circuittuned to y.approg;imateiy `120 oyolos- .flihis circoli .comprises t s costano@ presented .by :the windings of ...former .301| and tho capacity of aoondonsor 2.0.

This circuit isiri series with :o .rootiior 20.3. preferably of tho full `WoV-o ioridso tros. The D. -C--ooiout rathoof this rsoforrreferably ,ooo-

tains a t.smoothing condenser 2.04 .and f'fosds the Winding .of relay .1.08, as rnosviouslir explained- Tuned y.circuit 26|. 211.2., while yoiorins o :comparatively lovt7 .impedance to .the 20 -cycle l,cprrent Vcoiziirig .freni the switchboard, yet ,will rgpresentisuch highimpeda-nce to ivoice3signaling currents -upon the ,line `)as ,to impose a negligible load -yvith respect to such voice currents, this point being an important one because V-of the factathat the :tuned circuit is at all --tirnes bridged across `the transmission line.

The operation o f relay |08 will cause armatures and ,.Ztliereof ;to disconnect fthe switch.-

board output trom the line fand -will cause -theni to connect oscillator .1.0.6 to the outgoing line, the reason `for this Connection VSoaring sororiously been .explained io connection :with i s. ;1- Contact-at armaturo-offrolaviliis Joer. concocted to .To to. mioa1 205 Whoo the rootiiierfonit is cotanto .a completo assembly.,

conductor from terminal Lillyvill afford connection fito a contact :oi @relay fltl' and .to 12,0 cycle source )mtas illustratodzio Eis. 1- Contactar armature 3' is connected to two points, as indicated by the respective legends in Fig. 2. One connection is to the' cathode of certain tubes in receiver |04, for. purposes which later will be explained in connection with Fig. 4. The other connection from contact 1c is extended through a suitableV resistor 205 to the screen of the tube in oscillator l 06, for reasons later to be explained in a detailed discussion of this oscillator.

Making reference now particularly to Fig. 3, the portion of this ligure tothe left of the dotted line represents an oscillatorof the mechanical vibrator type, yielding a low frequency, such as 20 cycle. an electronic tube oscillator giving an output of voice frequency, such as 500 or 1000 cycle. While To the right of the dotted line is shown Y these two units are here shown as adjacent to one another, in order that their mutual electrical interaction may be more clearly understood, yet it is to be understood that such arrangement is not essential, but that the mechanical vibrator may be placed in any location desired, for example in the power supply unit.

The mechanical vibrator 30| has the driver magnet 302 thereof fed by the direct current derived from rectifier 303, this latter being preferably of the full wave, bridge type. This rectier is, in turn, fed from the secondary winding of transformer 304, the primary winding of this transformer being fed with a suitable supply of alternating current, as later to be explained in detail in connection with Fig. 5.

The output of rectier 303 is smoothed by condenser 305 and the completion of the circuit through driving magnet 302 of vibrator 30| is obtained only when the positive terminal of the rectier output is connected to ground, as will be apparent from the connections of magnet 302. The cycle output of vibrator 30| is fed into the mid-tapped primary winding of transformer 306. The secondary winding of this transformer is shunted by a suitable capacity 301 and resistor 308. Transformer 306 is designed to afford ay secondary output of approximately 80 volts at about 20 cycle, which output appears across terminals 300.

'I'he electronic oscillator of Fig. 3 includes a screen grid tube V5 which may be of the type commercially known as BGS-G, or other suitable type. This tube serves to excite a tuned circuit consisting of the primary winding of transformer,3|0, having a portion or al1 ofthe winding shunted by a capacity 3| |,suiijcient to make the circuit resonant either to 500 or 1000 cycles, as may be desired. The frequency may conveniently be determined by the operation of a switch' 3|2, the two arms of which are here shown as separated, for convenience in illustration, but which arms are simultaneously operable. This switch may be mechanically coupled with any or all of the switches utilized for frequency chang.-

ing at other points of the system. The anode supply for the electronic ltube is fed through the mid-point tap of transformer 3|0, and may be secured from the'powensupply" shown in Fig. 5. The oscillatory circuit is connected to the anode of the tube and, via a'suitable condenser 3| 3, to the control grid of the tube. This grid is likewise connectedthro'ugh two resistors 3M and 3|5, to the left hand terminal of the primary winding of 20 cycle transformer 306.

When the oscillator is not in operation, i. e.,\.

when the sending operators ringing key is not depressed, the screen grid of tube Va is grounded screen grid, keeps the oscillator tube in, arquiescent condition. Y Y Y When the sending operators ringing key is depressed, the 20 cycle current derived therefrom is rectied by element |05, and relay |08 .is vexcited, as describedv in connectionswith Fig. 1. The excitation of this relay causes armature 3 to leavecontact lc, thusremoving thelgroundfrom the screen gridof tube V5, and thus leaving this tubey and associated circuits freeV to oscillate, since the screen grid will then secure proper positive potential, through resistor 206', from the high potential anode rsupply marked +180.

Armature 3 then places-a ground upon contact y', to which contact is connectedv the'positive side of theoutput of rectifier 303. As previously explained, this completes the energy supply circuit 'of vibrator 30|; thus setting the vibrator into operation. l

When the vibrating element 3|| makes connection with the Vleft hand contact 3|8, aconnection to the grotmd.y is thus afforded for Vthe control grid of tubex-Vs. The lelectronic Voscillator will then besetinto operation at a Vfrequency of either 500 M1000-cycles, accordingto the position assumed byfswitch3l2. `When `vibrating element 3H makes contact with the right hand terminalv 3|9, current flows: from the ground on the vibrator through the right hand portion of the primary winding oftransformer 306, through rectifier 303, throughcontact y' of relay age lbetween the outside primary terminals Vof transformer 306, which acts to impress upon the grid of tube V5, a negative biassuicientlygreat to stop theoscillation offthis tube, during the output will be substantially constant at either 500 or 1000 cycle. level so that it will' be either zero db. or +6 db., these particular power levels being purely illustrative. The details of'such networks are well known kin the art, and therefore detailed description thereof is deemed superfluous. I l

A rectier 320 is shunted betweenthe connection point of resistors3|4 `and 3| 5, and the ground..V This rectier functions to smooth out irregularities in the modulated voice frequency output wave, for example those which may be produced by the effect of the 20 cycle vibrator contacts.` The final modulated output atvoice frequency appears across terminals 32|, which Y are extended so as to connect to contacts gand z' of relay |08, as shown in Fig. 1. f

There are indicated in Fig.y 3 certain cathode heating circuits, by-pass condensers and other y elements ancillary tothe operation of 'tube V5. Such. elements are well-known in the electronic tube artv and a detailed discussionrthereof isV accordingly thought to be unnecessary.

Reference is now made especially to '4j which illustrates the receiving device" used.v to

|08^to the ground. This generates a volt- 350. (preferably coupled to' switches 3|2), as indicatedin Fig. 3, sothat the Switch 35| changes the power va incalmofoycie,orzsimimralowirequency current, vwhich latter .rcnnzentziisi capable :of lactuating fthe .comfentioual .iringing'ssignal apparatus V.fdesign'ed for operation by such low frequency.

i lhen'eceivingzapparatnsiherefshowniisfcap'able of 'receiving:iaivoice flfe'eqlrency :signaling .current of euriterrmitedzzorzunmodulated character, "but is likewise :responsive rtoithe signaling rcurrentsof voice kvfrequency,-Zxrrodula'ted at 20 .'cycle, :such .as that producedihy theroscillator-.unttof Eig.v 3. :1n some :instances may .be desirable 'to use alnonmodulatedrirzging signal current "with an'inger according to thisiinventiongrand the 'receiver foi Eig. c4 accordingly iexible iin :its reparation, f'so as to :utilize :either type of eringingfsi'gna'l.

.ans :may :heissen ifrom )Eig .5rd, 'Itho-receiver is bridged `fiascross the line aiming :speech transm'ission fand accordingly it fis. znecessary to' "provide means iefiecizively` preventing the operation for the lreceiver'*dure tothe'ipresence `during :speech transmission of random frequencies, corresponding :toithe :500 'cycle :or 1000 :cycle 10i otheepredetcrminediirequencres usedioraringing .or'rforo'ther signaiing `ipnrpose's. the :point :of l'origin of such speech `frequencies may `ebe isituated, from an `electrical viewpoint wery near rthe receiver, theienergy :due Ito such: speech frequencies `may be'equel --to, or evenmuchigreater than,1the miniinum i-energy required'to fop'erate the z're'c'eiver, especially'when the datterfisadjusted :to Aa degree of :sensitivity :sufficient to sensible iits operation over :a relatively :long ,"transniission ilin'e.V Likewise, :fandom :pulsesiotshortfdurationtfrequently occur sin fsuoh itransmissionvsystems, es, `for example'from "the :operation fof a. switch -hook or the like.

zine order "ifto snake fmore certain that the receiver will respond only to'fthe desired ringing signals, the receiveriemplo'ys:eJ number-of devices to differentiate :between the vlrandom 'impulses above iueferred to `fand `Lthe 'intentional ringing signals. ln'tnn'e 'delayielement incorporated zin the ireceiveufhelps' -lto :secure f such @selective actuation of 'the .vreceiven :but has :not vin Yitsel 'been found in `:practice `Sto libe #a -su2fiicien`t fsa'feguar'cl for .the prevention fof lundesired lactuation 'of the receiver. l

Theireceivingunitillustrated in Figfll fac'c'ordtime cdelayrcircuit above referred to, but is arranged alsolto incorporate za'two-path 'transmission network. One path fvery 'sharply Ituned to :the predetermined ringing -frequency, jwhile the 'other path iis Idesigned to ipass substantially all the remaining -irequenoies 1which Emay lbe present :upon'the y-line, fe. g. 'the other ivoice transmission frequencies occurring eduring speech transmission. Thev respective i-outputs of ithese two mat-hs are -l'suitably and individually detected and then :arie diierent'iaily combined, rin such manner thanan'ampiifier tube fed by this combination ofboth the detected-signais will-respond only when the fgreater portion foi 'the energy which it receives lcomes lover the sharply tuned transmission path :ofithe network, i. e. when 'the energy 'iis concentrated about `the single ringing Y frequency.

'It :has been foundethatzthe' arrangement above described, including 'the time .delay Jcircuit, "will act iin 1an fextrernel-yrsatisfactory Sfa'shion 'in preventing false'nperationpi the rreceiver, :even when suchireceiver sitas :a degree iof .sensitivity su'iieientito respond :ringing fsignals transmitted over :ahoutg rlbnf transmissionline. 1 i

".he .unput fto idervediromxicom tactsfa and ofirclay I lfeedsithe primary Winde ing of `transformer SMH. Sinoe 'this primary lis bridged across the transmission line 'during speech transmission, the transformer fis'preferably Emade of high input impedance, in order 'to avoid undue 'bridging loss fior speech transmission. "It has 5rbeen f'found desirable 'to include condenser 40i" in 'the primary circuit, in 'order to prevent a metallic bridging ofthe transmission line. 'It-'also fhas'been found 'that a transformer with :an impedance Vratio of 1`0*,000:500,U0`0 will impose 'negligible floss upon the 1ine,'but other values may vbe `used. The "secondary vof ftransformer 'fd [El is 'shun'ted Eby rresistances @U2 land 54H3 connected in series. 'The gain control switch #464 is 'arranged A'as indicated, so lthat the effective output jpoten'tiafl of tmnsformer lill can be chosen'to vassume'three different relative values. When fsw'itch WM vis placed on the point marked High, thema-Ximum-output of the transformer is available, such selection being made when the receiver must :be operated over Aa transmission line of such length orother characteristics -as "to make the highest degree of 'sensitivity .at the receiver-advisable. 'Whensw'itch '.604 is'placed on the position vmarked Low, the sensitivity 'is reducedby'about l5 db. The `point marked Zero is used-whenit is desirable -to prevent operation of the receiver, rsuch 'condition being encountered, for example, whentestingis vbeing done `upon 'the receiver itself, or Yupon the transmission .line to which it is connected. AThe output of transformer lll'l lis vsuitably ampliied'by an electronic-tube Vi. The usual .energy feeding .and'bias determining elements .associated with 'this tube are .indicated .in Fig. 4, .but a detaileddescptionlthereof is thought unnecessary, as suchelementsare well-.known in the art. The output .of'tube V1 is divided through a. .suitable resistive .network including resistors 405 and 406 so 'that a portion of the .energy `isffed to the two respective .trans-mission paths, .above described. In the drawings, :theerrows markedA and B, re spectively, show these vtwo transmission paths,v path A starting at the end of resistor VVdilliconnected 'to the vanode -of Atube Vi, while path B iarts `.at the junction 1:between resistors 505 :and mransmission rpath A and all ,the :elements thereof,.except ...the switching devices, are ,preferably `enclosed 'in a shielding -container 401 indicated :by dotted lines. This shielded transmission ,network includes inductance and capacitance elements forming two sharply tuned circuits, the coupling elements necessary to transfer energy from one circuit to the next, sand an ,output load resistance for .transferring the output to the 'detector tube V2, fed by 'the energy received over transmission path A. Voltages appearing across theoutput terminalsioffthenetwork 'are impressed upon the load resistor'MS and ,upon the igrrdcoi detector tube V2. 'Ihe frequency vs. voltage `characteristicacross the network output terminals ,is shownby the appropriatecurve in Fig. 7. fFig."l also ishows the curve'of frequency-voltage rwhich appears across the network input terminals. The voltage appearing `across the network input terminals Ais impressed ,in vwhole or vin :part upon theggrid of detector tube V3, through the coupling condenser 2408. The .ratio ofV :the .resistance of 495 to 466 determines whether the whole .or 'only a inaction ofzthevoltagenppearing across .theinput terminals '.is impressed :upon *the grid -zof Vs. 4.The reasons :for impre'ssing upon tpath .-B only =a tfra'caseaeie tion of the network input voltage will be later given.

It can be seen from the frequency-output volti age curve of Fig. 7 that thel voltage applied to the grid of V2 vla path A will be substantially at a maximum for. a narrow band of frequencies grouped around the signaling frequency and the output voltage will have a reduced valuefc-r all other frequencies outside this narrow band. Itl

will also be evident from the characteristic frequency-input voltage curve that the voltage applied to the grid of detector tube V3 via path B will be at a minimum over a narrow band lying near the signaling frequency and -will have an increased value for all other frequencies lying outside this narrow band. Suitable switches are provided for changing the frequency discriminationfrom 500 to 1000 cycles, or to such other voice frequencies as may be employed. These switching devices are indicated by the numeral 401', and they may be mechanically coupled with switches 3l2 and 350, already described in connection with Fig. 3, so that one switching operation will change frequencies throughout a complete unit.

Transmission path A acts, then, to accept the principal portion of the energy entering the receiver at the predetermined signaling frequency, suchas 1000 cycles, and substantially to reject all other frequencies. Currents passing through path A are detected by electronic tube V2. Condenser 4|8 and grid leak resistance 4|9allow the proper rectifying action of detector tubeV2 to be obtained. Transmission path B, on Vthe other hand, due to the decreased voltage across its input, acts virtually to reject a small band of frequencies around `the predetermined signaling frequency and to accept all other frequencies' outside this narrow band. Currents passing through path B are detected by electronic tube V3. Condenser 408 and grid'leak Vresistance' 409v secure the proper rectifying vaction of V3.

Transmission path B functions to prevent operation of the receiver from voice currents in which random frequencies corresponding to the predetermined signaling frequency may be present and may be termed a guard circuit.A In this invention, the inductance and capacitance elements in the four terminal network serve the dualpurposeof providing a band pass frequency discriminatingenetwork for path A and a band rejection dising advantage of both the input and output volt- V criminating network for path B. By thus Atakage characteristics of the network, the network elements of inductance and capacitance yare employed to secure the desired results in an economical manner. This constitutes a definite improvement overother proposed signaling systems in which one set of elements is required for the band pass path and a separate set of elements is required for the band rejection path. necessary relationships between vthe various sections of the network and the manner in which the sections may be adjusted to obtain the desired input and output voltage characteristics will be later explained.

The anodes of electronic tubes V2 and ViareY coupled through a resistance network comprising' fixed resistors M0 and 4H, together with variable resistor 4|2. The values of resistors 4U and M2 are such that the total resistance presented by the two in series may be adjusted so as to be equal to that ofresistor M0, or so as toV differ slightly therefrom, for reasons hereinafter to bev explained, Thel anode power supply is fed to The' ' equivalentto still further decreasing the .negative 4H and is here shown as derived; from the power supplyunitfillustrated in'detail 'in Fig. 5. This the connection pointl between lresistors '4 t0n-and powersupply unit is also indicated as furnishing the other necessary voltages to various elements of the receiver,

-Thefend ofl resistor 410 nearest the anode of tube V2 is likewise' connected to the control grid of i an eutput'tube'fVf; through a suitable resistance M3, whilejthe anode end of resistor 4I2 is `also connected; through f cathodeof tubeVLi. The diifere'nce in 'thevvoltage drop in the anode resistors connected with; tubes V2 and V3', respectively,` is in this fashionf bridged vide the requiredtime delay action Vin the operation of relay |01. Thistime delay istovprevent false; operation of relay I 01,* aspreviously described; Y Y f A f eed backcircuit-comprising ,a resistor 4-I6 andacapacity 4| 'i i n'series, is provided'between the anode of tube V2 and the control grid of tube V2, for a purpose hereinaftertojbe explained. This circuit may alternativelyextend between other specific elements -of the tubes Atrbe -so coupled;'e g.j from theggrid of V2 to 'the anodeQof V3,A as Ys hovvnj in AI"igj 4 a) The, usualV voltage dropping'resistors andloy-passV condensers` used in conjunction with the proper excitation -of Aelectronic .tubes V2, V3' and V4 are indicated, as well as cathode heatin'gjcircuitsjfor these sametubes.

The'lenergy supply fo r, suchl circuits V may conveniently besecured-from'the power Ysupply unitl shown inlig;r 5^and' it is thought'unnecessary to discuss in detail these elements well-known in the electronic art. 1 g f The operation of.this receiverjisjbest; understood by first consideringithewaction when a ringing signal of vvoice frequency is beingreceived, and

with the assumptionthat no other, interfering frequencies` are presentl upon the, transmission "..linei Undersuch conditionsmhemajor amount of the Vreceived energy will be transmittedthrough path A (denoted by arrow n) and will causeth'eanode current offtubeVz to decrease in value, thusi increasing the 'potentialat thejcontrol grid'of tube V4; by making the j'gridjle'sg negative.y This; Ainiy turn, increasestheanodeiiowtotube V4, through the winding of'relay 101, thus operating this relay.

The feedback path, comprising elements 4 l6 and sa1;.,wi11thenninction to lmake the contrai grid on Ai'fube/'s' more positive, ythus increasing the anode current of thisztube and thus tending to drive the cathode .of tube V4'less positive, which is b ias on the .control gridfoftubejVi,y thus affording are-generativel ,amplicati'onofthe effect, already produced directly in tube Valby the changejof lvalue of the anode current of tube V2.x v Y The operation ofthe receivermaynowjbe considered when speech l transmission. is: occurring" upon the line, duringjwhich time rando-rn frequeng ces'sorres'pondne .toithe-predetermined;,rinsing Y frequencies may'be' encountered;y @Undery such' af suitable resistors I4 tov the V conditions. energyr will. heetransmitted. thronglnv botnpaths., fand-B., The cnergyrpassinggthroilgbs path-A will be substantiallyrsolelitthat at-.the-nree determined ringingifrequenoy... while: tice-- encrayq. missing timone-.hz pathgB. willu befthat. at substan-v tially al1 the other speech frequencies present. upon-.the,line, aswell as some; random energyfat the ringingfrequency., It.,wilLbeev-ident,that .thee energy; represented` by the; sum otY tha-frequencies? other than= theringing frequency, will bamuch.. greater thanlrthe; energyv` passingg through. nath A. 'llhisibrings it about that.detectorftubefvawill une dergo. a.- reduction.y in.r anodeI current.. which; wi1l; in. turn,l increase the potential.- upon? thecathode; oL-tubcla.thisbeinquivalentztovanincrease.in.

tending toreduce: the.r now of, anode currenti through. relairf` |041., Y This. effect. brings it. ahout, then, that. under-these conditions; relay. |01 will- 5. Formula1` the negativ e grid. bias. of.. tira-,latterl tube,

determinedsignal. frequency, the.: theoretical. valeues of inductance and capacitance for the inputs section:` is.- giyen-.bntherelation:

l. 21N/` L1 In this lfm1-mula. Li.. and, C1L referto. vthe inducteance; andy capacitance elements of. the tuned cir.-v cuits o. Eig,.4.. C5 represents.theinputcoupling 10icondenser. and Liathepredetermined signaLfrefluency). When, this.` relationship; is. satisfied, the

reactance. of the.. first. section;z looking.l in .atithe.

input:terminalsiszero..

EvenL though .the input reactanee,ismadeemml` 15; to.L zeroi. an undesirable; voltageatthesignal fre,-

fluency. maglie. developed. across.. the input. ter.-

minalstifethe rstcoilothenlter. has appreciable. resistanca. and of courseany. `physical coil, must.. have: some. resistancaA In. order.V to. mitigate the.

maintain.thearmatures,thereoi` retained. in thein. effectslof any residual. voltages of. predetermined respective7 open positions.. since.. thea effect.- ot the energy transmitted through path; B( reducing the anode-currentofcVi. willfbesmuchereater. .than the; effect ofa .r.elatively smaller amount, oensignah frequency.A appearing; across. terminala ar-b.` caused: by, resistance. in the. coil; the., eiec:

til/'ei gaini ot, the, gnam.y circuit i is reduced bythe potentiometer arrangement.. of.. resistors .4&5 Vand ergyfpnassing` through nathA-whichlatter energy, das. This allows only a fraction of the undesired.

would .tend to-.excite.relay, LUL

The, cathodesfof. tubes.. V1.I and Vc.. are. normally groundedthrough contactJaat armaturc` oirelay;v 1118:. When. a signal .isibeing transmitted. onthe; line, this relay is operated, as described in con.- nectionwith Eig, 1 andVY the ground. is removed from thefcathociesofthesetwo tubes, thus render-V ing` the .receiver inoperative. while; such signal is.. beingy transmitted.

A negativebias voltageonlthefgrid of V4 .islequiredso that; substantially no .plata current. will flow. throughtherelay winding when no.sig;nal beinggreceived. Inthis .receiver-,the required'grid; bias is obtained by. adjusting thevariable. resistvolt-age; to.. become-effective. in.. the,guardl circuit. The resistanoeratioof dftoililiaadjlflsted so. that thelfraction .ofi the .undesiredvoltagaotinre: determined signalrfrequencu appearing across the,

3m input terminals .is,not. great enoughto -cause anamzgrecialole change-.in ,theplate current of; Vs. when, the.: signalrfreqllency' is f. applied to the r e ceiven. Y

Ilhe v requirement., that, the, output ,voltage shall be. maximum.` at. theJ predetermined signal ire,-

quencytcanbametbyaverci-f Eormula.2.

anceV M2. so;I thai-, thereA will. be: a differenceoff 40s wherelaCarefer.toftheelements .showninlFigi 4.

potential between theplateoizfand the plateofl Va. and.. in .such` direotionthat the; plate; of. V is. negative. withirespect.. to the plate. of. Va, adjustment is. made at atimeewhen. no signals offv and. f. is the: predetermined signalfrequency.

In Formulae Land Z, the.` assumption is made thatwhenthe inputandoutputsections Yare connected togethenby couplinaoondenser C that the any description.- are being receivedfrom the 1ine.A 4in separate. performance of, the ,input and output The: bias voltage. developedbetween .the plateao" V2 and Vs is applied to the grid and'cathodeofm through resistorsA l 3 and 4l 4.

One of. the requirements, for. the networkv is sectionswill. net .bemodifieditd any, great extentA by.. this coupling. Thi`s,will ,be substantially. true inthe.reactance of;C3.is made very large. by. com- Iiarisonto, ther other. reactances in the.. network,

that, the4 impedance looking.. into. thenetwork at' 59,1113.V was. alsol assumedthattheu reactance of; con.-

signaling frequency., the voltag'eacross .suoli ter- 65a minals .will be, negligible, atthis frequency. This isa. necessary condition in..or.der. that voltageoi' the predetermined.signalv frequency willnot;be applied toly the. guard circuit. and. prevent. proper operationfo.the.reoeiver, andlisfshownsatnoint' 30g been. chosen and designed S0.- tha.the inputr re:

lllsofacurve 'IUli.`

Another-.requirementor the network isttiat the.

frequency output, voltage, characteristic should reach a. maximum at the@ predeterminedsignal" denser. Ceisinadevery smallby comparison to actance. is. substantially zero. at the predeterminecL signal frequency., Inallqcaseshadvantasc is: taken .of.the-peculiar frequency vs. inputvolt agacharacteristics of 'thenetworntoactuate .the

frequency asfshownatpointlllz of tha curve 10]; 6.1 guarlicircuit.and, simu1taneouslyadvantage is oi Fig. '7.

The network shown,in Fi"g4.-fulll`s tlie above requirementsina satisfactory. manner. ifgthe relationship between. the,networlneleznentsL are. satis: fied as described below..

Referring.tolig. 4..the.network mayheonsideredior-convenience,ofexnlanation tu.havetl'uneV sectionsaswilllhe,anparenttothosaskilledin the..

art.-v In .order.to .satisfy. the reouitementthat, the, input. inmdance. shall. be. minimum. at.y theipree taken ofthe frequency vs. output VvoltagecharacteristicsN of the, same .networks to oneratevacuumtubaVt inpath A.

Itgwillibe. notedftliat' the reoeiven here. described 7m isnot. dependent; for itsfoperation. uponi the utilization'oi amodulated. voicerrecuency rinsing.. Signat. :l.1rr,e r 1t'.', butA willfbe Operated' loyfaY continuous, ringihgslgnal current. at the Qredeterjmined voice frequency; While the presence of` Media20; cycl`e modulationv desirable' the' special feedback circuit from the screenv grids of tubes V2 and V3 to the anode Vof tube V1, yet it often is desirable to use such modulation when employingl this invention inv conjunction with certain other signaling systems which lattery deemploy such modulated voice frequency ringing currents. The fact that the receiver will operate on such continuous signaling current aiordsthe further advantage that it may be employed in transmission networks employing such modulated voice frequency ringing currents, evenif, as frequently happens, the current intervals due to the modulation tend to ll up from reflected currents on Athe line or from other causes. Atlow power levels the 20 cycle modulated current may tend to cause chattering of relay |01, but the feedback action caused by the fact that the screen grids of tubes V2 and V3 secure their feed `from the anode of Vi, instead of directly from the power supply, tends to remove or minimize such diculty. f

To change the receiver so as to operateonusome other predetermined ringing frequency, it is mere- 'ly necessary to change switch 401 from `one position to the other, which will change the inductances and capacities found in the various filter elements disposed along the interstage network of transmission path A. As previously explained, the oscillator frequencyshouldbe simultaneously changed, which mayreadily be accomplished by a mechanical coupling of. switch 401' to switch 3I2 (Fig. 3). The oscillator output will, of course, at the same time ybe suitably changed by suitable adjustments of the network 3I0, as by changing switchv 350 thereof, as shown in Figs. 3 and 6, in a manner which will be apparent to one skilled in the art. All the switches just mentioned may be mechanically intercoupled, if so desired, to facilitate switching operations. u A

The manner in which the operation of relay l'l by the receiver just described, causes the actuation of the drop or other signaling device at the receiving switchboard, has been described in connection with Fig. 1, and, briefly stated, con sists of the connection Vof a supply of 20 cycle energy directly to such signalingdevice. AAt the same time it will be noted that the line is disconnected at armatures I and 2of relay I0?, soV that undesired energywill not be transferred back to the line. While not being confined to the use of any specialV type of electronic tube, the type commercially known as GGS-G has been found suitable for use as tubes Vi, V2, V3 and Vi of Fig. 4, when the various elements there shown 'have theA values indicated. u

In Fig. 5 there is shown a power supply unit used in connection with a complete ringer assembly, according to this. invention, and alording all the electrical energy required, both A. C. and D. C., and of various frequencies and voltages, for the operation of such complete ringer assembly. eration from a commercial A. C. supply of any conventional voltage between 100 and 250, although by suitable changes apparent to those skilled in the art, energy supplies of other type may be employed. As one alternative supply, there is shown the necessary switching and conversion apparatus to enable the power unit to be operated from 12 volts D. C., such as may readily be obtained from a battery or the like.

The power unit supplies its output to a series of terminals shown at the right sideof Fig. 5 and numbered to I0, serially. -.'.eruriinalsjl This power supply is adapted for opf and 2 supply 180 VvoltsD. C. with the negative sidel grounded. Terminals 3 and- 4 supply v'l5V volts D. C., free from'any ground connection, asis necessary for the cathode-anodesupply oftube V4,

shown inV Fig. 4. Terminals'S and .16 supply 6.3 volts A; C. for the cathode heating circuits of the various tubes employed infthis invention. Ter- -minalsfll and `8 supply the 20 cycle ringing current at. approximately volts. Terminals 9 and I0 correspond with those similarly numbered in Fig. 3 and afford connectionA between the voice frequency electronic oscillator shown at the right of Fig. 3 and the mechanical vibrator shown at the left of Fig. 3 andV also at the bottom right handofFig. 5. y' l L A transformer304; of which a. portionV is indicated at the extreme left of Fig.' 3, is .supplied with va plurality'oi` windings. One 'terminal'SIlI is connected through a suitable fuse 502 andV switch 503 to a common terminal 504. yVarious taps 505, 50,6, 501 and' 508 are taken fromrone primary winding v500 of thetransformer, andare for connection to the various mains-voltages rirl-'- dicated in Fig. 5. The throwing of switch 503- tovr theA left will then energize'the'primary winding of transformer 304. u 1

Secondary winding50I, bridged by a suitable capacity 5II, feeds rectifier 5I2the D. C. output of whichis ltered by capacities 5I3 and z5M and inductance SI5, Vbeing delivered to output terminals I andr2. f y Y The output of secondaryfwindingv 5I6, similarly is-rectiled -by element 5I? andfiltered by condenser 5 I 8 and resistor 5 I 9, theoutput appearing on terminals 3 and 4.`

- Secondary winding 520, supplies 6.32voltsA. C. for cathode heating purposes and is provided with a suitable regulating resistance 52|, controlled by the action ofswitch 503.v f Y .f

Secondary-'winding 522 is identical` with the secondary winding shown at theleft of Fig. 3 and the rectifier, lter, vibrator, transformer and other ielements shown connectedto winding `522 of'the dotted line vand bear identicalreference numerals. Asexplained inconnection withFig. 3, vibrator 30| serves not only to supply 20;'cycle current at terminals 1 and 8, corresponding with terminals 309 of` FigfB, but also Vacts to determine the modulation of the electronic oscillator shown at the right of Fig. 3, by changing the negative bias thereupon, Vs explained in connection with the description of Fig. 3. I Y

To operate the power supply unit vof Fig. v5 from'a 12 volt D. C. source such as a battery, switch 503 is thrown to the right. f Thisconnects the battery Vwith a mechanical vibrator 523, via a suitable capacity-inductance filter system 524, as is well-known inthe art. Theoutput Yofvvibrator'523V is fed t0 a mid-tapped pri-rv mary winding 525,0f transformer 304, thus exciting all the secondary windings of the-trans. formerin the same manner as previously described when primary winding 509 was excited;y from the mains voltage. The connection of theY power supply unit ywith they other units'of a complete ringer may conveniently be made by connecting 'terminalsy I-I0 to a multi-terminal socketwith which a suitable plug and cable mayl be interconnected. I

In Fig. 6, there areshown', connectedtogether;

a complete set ofV the .various units except the andrecepti'oni. terminal'smtithetlowerfright lia-nu' portion; on Eigicare numberedcorresponda ingly'with. the? terminalsi alithefrighiiside off-the' powerrsupplyfurutrshowniineFig 5; beingeintendedi for connectiorrtherewithf, althouglianyfothersuit able sources. ofi powen mayJ employed" instead` ot the powerfunitliereshowin Intriga-enhance"- work 3110: is shownwitn oneform ofiswitchi-nedevice whichmay befvemp'loye'di to givethe variim ousadjustmentsneeded forai predeterminedfringiing-ifrequencyL or' poweroutput level; asdiscussed ini connection with Figi: 3L butt othenfcrmsf. may

heuse'd..

The remainingz elementss shown Eig. 6i cor-f respond with those already shown anddescrilied'l irrlcormect'ion. withiEigs-z inland" bearrefe'rence mnnerals identical with tiliose alreadyfemployed suiv that! ai detailed descrlptioni thereo'f wouldf he repetitions:` l

It'l'is `possible :to adapt :this system? to'l signaiingA systems? other.` chair ringing; forv example@ longY distance telephone automaticswitchingf and? thea like.; Many-ofi these. long-f distance schemesz re quire.` a: receiverzf with'l a guardcircuit 1 that4 canq beibri'dgedfacrossthetalking line; as is'dclne"i in' the case of the instant invention;

Certain'i portions'. of systemsa-lready linown'- in thei art and designed tc=perfrm1the same: func-4 tionsias-theinstantsystem, may-additionally he' ix'icorporated1 therein. Forinstance;` addition--A :illal llter, tuned to reject the predetermined-fsig=v naling frequencyvmayfbe1inserted'inftransmission path stills further tor reduce theL transrnission` therethrough` offsuchisignalihgfrequency.A

Other changes and additions inoursystemwillbe apparent' to fthosef skilled initlie* art', and'may be made v without departing 'from-trie xscopel offthehereuntoappendedclaimss What is claimed is:

1E Af signal lreceiver having-{the' ability# todiscrimiiiate between asignal ofi a---predeterrnined frequency-i and-'- speeclr signal'sincluding said 'pre-v determined' frequency; includingv meansf forfreJ ceivingiallsaid? signals; two transmission paths;`A thefrst path sharply tunedtosamsignalof pred'etermined?I frequency and the second path: substantially aperiodic,I meansl fory dividingL theE received signals betweensaid two-paths,A` wherebyJ onlyr saidsignali ofi predet'errnin'edV frequencyA passes` toiaamajorextent over `l`said'i first patri and? all :other:` frequencies passltoramajorrextent over said secondpath; means-'for combining the'out'r puts: of; said paths; to'rpro'duce the diiierenceV` le'- tween said outputs,zmeansfiirf determining'the'v closure ofi ai relayfwhen the 'f' maj orfani-cunt foi fenergyftransmittedpasseszover said first lpath andl meansforfdetermining the opening of saidrelayf' when' the major: amount: of: energy'l` transmitte di' passesovensaidrsecond path;A i

2:'. In:v signaling; systems'i/carrying; both. speech-1 signals andfan'other. signal'l'oa piedec'errnin'edzfrevquency lying; within; ther range; of; speech fre;i quencies; a receiverfresponsive"l only tosaid other" signalnY comprising a: two-.channel transmissionpath; kmeans for: directing the: majors' portion-v ofi said othersignalroversthe first channellandmeans fori directing;A thenV majorb portion: ofy thelspecchi signalsroverrthefsecondchanriel, ai'. deem'odulator at4 the-termination of each channel; meanslforf combining the; respective; rie-modulated outputs; 7m of both channels to produce the diierencefoffsaid'f outnutsr. .relay` meazns normally `unoperated'- but capable-offactuationbythe comliinedloutputsandl responsive-to.other:relativeienergyflntheirespectivei out'mltsn; whereby: greaterf energy? inl said irstfi till' accusata channeh will operate saidg relay means, while greater energyrinfsa'id second channel=will'rn'ainr tainsaid!relaymeansfunoperated;

3S Device according tirclaim` 2,' inv whichsal'df other' signal' 'andi speech signals directive means includes an electron relay, the combinedsignalsl beingffedto the gridthereoffand including ay cur;V rent dividing networkv in' the anode circuit` there# of; said respective channels being connected to di'erentpor-tions of-` said anode network.

4. Device according'to claim` 2, in Which said' rst channelf includes aV resonant network' tuned* topass:substantially-onlyfsaid other signal,` while saidfsecondchannelpossesses substantially aperin odicSpass-characteristicsspense; but' the intercorrne'ction of saidlt'wo channelscauses-said second channel to have virtualband'rejfection characteristics:-

5. Dev-ice". according toclaiznZ; in whi'chsai'cf relay means 'includes an electron relay 'having t an: electro-magnetic relay in the anode circuit there; oi'-, saidlelectrorr relaybeing normally negatively biased t'orredce the anode current suiciently to' maintainssa'id electro-magnetic relay in'non-operative-vposition; and means actuated by saidrespective outputs whereby greater relative energy. in said-irst5 channel will decrease said negative' bias, increase-said'anode current, and operate said electromagnetic relay;

6l Al ringing signal receiver"including'.meansfr'siinultaneously injecting therein a" ringingsi'ginalof` predetermined frequency and speech sig.- nals; means yforsubsi-emtially'separating saidri'n'gf' i'ngfsig-nal from saldi speech signals. includingal first transmission path having highV acceptance substantially solelyA for the ringing signal frequency; means forpassing'the'speech signals including a second'transmission path having ac.- ceptance-.for a'broadra'nge' of frequencies, means? for combining the respectiveoutputs of both saidpaths to producecthediierence of said outputs and relay meansactuated by said difference; only when the energy passing through said iirst path` i's substantally'greater than" the energy passing. through said second path.

7.' Device' according to claim 1, in which said means 'for differentially combining the respective. outputs of-both'paths includes Vtwo detector tubes, onefed fronrea'ch of said paths;.a resistance net- A'work' connected tothe anodes of both tubes, anv

output tube having the' grid and cathodethereoff connected4 to' points on` said network so chosenY thatwhen no'si'gn'als are` received, said grid will be negatively biased, whereby. signals received. over 'said rst path will tend'to drive saidgrid positive and actuate said relay means, while signais received oversaid Vsecond'patlrwill tend` to maintain'said grid negative.

8i Device accordingto' claim 6,'.wherein the.in put to saidrneans-for differentially combining theV outputsof 'said paths includes a resistance-capacity'networkhavinga time constant suiiiciently'great to4 prevent' response to received signals. of relatively short time duration.Y

9i The methodof receivinga sign-al of'a predetermined 'frequency 'when the line carrying 'said signal isa'lso'carrying speech signals including. saidfrequency; which includes the step ofidividingfthetenergyfrom said line into two channels, attenuating all frequencies except said'predeter` mined frequency in the' 'rst channel,v passing' Vsubstantiallyall speech frequencies' except those proximatel to vsaid-predetermined frequency, with uniformJ attenuatiorr through thesecond chan-"-A nel,Y combining the^-outputs of saidchannels to produce the diierence of said outputs, determining which channel is passing the greater amount of energy and actuating a detector for the signal of predetermined frequency only when the greater amount of energy is passing over said rst channel.

10. Device according to claim 2, and also inciuding a capacity connected in shunt to the input of said means for differentially combining said de-modulated outputs, whereby response to said relay means to transient frequencies is hindered.

11. Device according to claim 2, in which said de-modulators are electronic detector tubes and in which a regenerative feed-back path is supplied from a first element of the detector tube of said first channel to a second element of the detector tube of said second channel, said second element being opposite in phase to said rst element, whereby the differential of the respective outputs is increased, for a given input to the grid of the detector tube of said iirst channel.

Al2. A signal receiver including means for simultaneously injecting therein a signal of predetermined frequency and speech signals, means -Iv for separating said signal of predetermined fren quency from said speech signals, including a first transmission path having high acceptance substantially solely for the signal of predetermined frequency, means for passing the speech signals including a second transmission path having acceptance for a broad range of frequencies, means for combining the respective outputs of both paths to produce the difference of said outputs, including two detector tubes, one fed from each of said paths, two resistors of approximately the same values of resistance connected one in series with the anode circuit of each tube, means for varying the value of one of said resistors so that the normal relative potentials of the two anodes can be adjusted to be slightly diierent from one another, a common feed for the two anodes and van output tube having the gridk and cathode thereof connected to the respective anodes so that when no signals are received, said grid will be negatively biased, whereby signals received over said rst path will tend to drive said grid positive and actuate said relay means, while signals received over said second path will tend to maintain said grid negative.

13. Device according to claim l2, and also including two resistors, one connected between the anode of one of said detector tubes and the grid of said output tube and the other connected between the anode of said other detector tube and the cathode of said output tube, and a capacity shunting the cathode and grid of said output tube, whereby relative persistence of input energy is necessary in order to actuate said output tube.

14. A signal receiver for receiving both signal and speech frequencies, responsive to a signal of a predetermined frequency, including means for discriminating against operation by speech frequencies which include said predetermined frequency, said means including two transmission paths and a reactive network for diverting the energy of a narrow band of frequencies including said predetermined frequency into the rst path, and for diverting substantially al1 other frequencies into the second path, said reactive network transmitting said narrow band of frequencies` with small attenuation and presenting a relatively small input impedance to said narrow band of frequencies.

15. Device according to claim 2, in which said de-modulators are electronic tetrodes and in which the screen-grids of said tetrodes are coupled back to the input of said two-channel transmission path so as to secure regenerative eiTects.

WILLIAM GIBSON STANLEY. FRANK AUGUSTUS LEIBE. 

